vertical bar

Vertical bar, or pipe is the name of the ASCII character at position 124 (decimal). The character is depicted as either a solid vertical bar ("|") or a vertical bar with a break in the middle (broken bar ""). The character is usually depicted as a broken bar on IBM PC keyboards to distinguish it from other characters. However, in Unicode the "broken bar" is a separate character, U+00A6 (¦).

Usage:

Mathematics

The vertical bar is used as a mathematical symbol in

absolute value: $|x|$ , read "the 'absolute value (or modulus') of x".
norms: $||(x_1,x_2)||$ , read "the 'norm' of x sub one, x sub two"; though Unicode also provides a special double vertical line symbol U+2016: x
set definition: $\{x|x<2\}$ , read "the set of x 'such that' x is less than two". Often sets are written with a colon ':' rather than a vertical bar.
conditional probability: $P(X|Y)$ , read "the probability of X 'given' Y".
divisibility: $a | b$ , read "a 'divides' b".

Cursors

In text that cannot be edited but can be selected, for example, on a web page, the cursor can also be a vertical bar, possibly with serifs at the top and bottom.

Wiki markup

The vertical bar is used in internal links to separate a link from the displayed text. E.g., [[c programming language|C]] displays as C.

Backus-Naur form

In BNF the expression consists of sequences of symbols and/or sequences separated by '|', indicating a choice, the whole being a possible substitution for the symbol on the left.

::= |

Unix

A pipe is an operating system mechanism originating in Unix which allows the output of one process to be used as input to another. Used in a shell command, this is represented by the vertical bar character. See Pipe (Unix).

Regular Expression

In regular expressions, the pipe indicates alternation. E.g., grep 'foo|bar' matches lines containing 'foo' or 'bar'.

Disjunction

In the C, Perl, and Java programming languages, among many others, it is used to designate bitwise or (e.g. a|b) or doubled up logical or (e.g. a||b).

Delimiter

Although not as common as commas or tabs, the vertical bar/pipe symbol can be used as delimiter in a flat file. An example of a pipe-delimited standard data format is LEDES 1998B.

Etymology

The name pipe for the character "|" came into common usage due to the fact that it is used in the Unix shell to pipe the output of one program into another program.

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